Embodiments of the present invention relate to compressed-air energy-storage systems, also shortly called CAES systems. The disclosure also refers to methods for energy storage and electric energy production using CAES systems. More specifically embodiments of the present invention concern so-called adiabatic CAES systems (ACAES) also named advanced adiabatic CAES systems (AACAES).
CAES power plants or systems are commonly used as a means for optimizing the exploitation of energy. As known to those skilled in the art, the electric power required from the electric distribution grid varies with peaks of electric power requests during the day and reduced power request at nighttime. Large steam power plants or renewable power plants produce an amount of power, which cannot be varied at will. This causes an excess of power available on the electric distribution grid at nighttime and a power shortage during peak hours. Small power plants using gas turbines, especially aeroderivative gas turbines, have been implemented in order to cover the peak power requests. These plants can be turned on and shut down according to the variable power requirement during the 24 hours. This notwithstanding, further measures must be met, in order to store the energy in excess produced at night time and recover the stored energy to increase the electric power production during peak hours. One of the means used for that purpose is the CAES technology. These systems typically include a compression train having one or more compressors, which are driven by electric power from the grid during night time, i.e. when less power is required than available on the grid. Excess power available from the grid is thus transformed into pressure energy of the stored compressed air.
The compressed air is then exploited during the day to cover peak power requests from the grid, expanding the compressed air to a suitable pressure and burning an air/fuel mixture in a combustion chamber to generate combustion gases, which are expanded in a turbine for power generation.
In order to reduce the environmental impact of these plants, so-called adiabatic or advanced-adiabatic compressed-air energy-storage systems (ACAES or AACAES) have been developed. ACAES or AACAES systems do not make use of fossil fuel to convert the accumulated energy into electric power. Rather, they store heat generated by the process of air compression and recover the heat to increase the air temperature before expanding the compressed air through one or more expanders.
In FIG. 1 an AACAES or ACAES system according to the current art is schematically represented. The ACAES system is labeled 100 as a whole. The system includes a compressor train 101 which, in the exemplary embodiment shown in FIG. 1, has three serially arranged compressors 103, 105, 107, having a common shaft line 109. Air entering the first compressor 103 at the compressor inlet 1031 is sequentially compressed at increasing pressure values and finally delivered at the outlet 107E of the last compressor 107. Between at least two sequentially arranged compressors, in the example between compressor 103 and compressor 105, an intercooler 111 is arranged. The intercooler is a heat exchanger wherein partially compressed air delivered from the upstream compressor is cooled before entering the next compressor, so that the volume of the air being processed is reduced by removing heat therefrom. Heat is removed by means of a heat exchange against ambient air, water or any other cooling medium. By removing heat from the partially compressed air the amount of mechanical power required to drive the compressor train 101 is reduced.
Compressed air exiting the compressor train at 107E flows through a thermal energy storage arrangement 113, wherein heat is removed from the compressed air flow and stored in a suitable thermal energy storage medium, for example a solid heat storage medium, such as rock, or a liquid heat storage medium such as oil, compressed water or glycol. In some known embodiments heat is stored in a heat storage medium which is selected so as to undergo a phase transformation from solid into liquid thus accumulating thermal energy in the form of latent liquefaction heat.
The cooled air is finally delivered through a safety cooler 115 and stored in an air storage device, for example a cavern 117. In the schematic of FIG. 1 exemplary pressure and temperature values of the air stream are indicated. These values are given by way of example only. Air exiting the first compressor 103 may have a pressure value of 7 bar and a temperature of 250° C. and is cooled down to 180° C. in the intercooler 111 before entering the second compressor 105. The air pressure is boosted up to 28 bar by the second compressor 105 and achieves a temperature of 450° C. before being processed by the last compressor 107 or compressor train 101, at the delivery side whereof the air may achieve a pressure of 65 bar and a temperature of 650° C. After cooling in the thermal energy storage unit air may have a temperature of 70° C. and substantially the same pressure as at the inlet side of the thermal energy storage unit, if pressure drop across the thermal energy storage unit is negligible.
The compressor train 101 can be driven by a reversible electric machine 119 which is selectively connectable with the shaft line 109 through a first clutch 121. The reversible electric machine 119 operates in the motor mode when excess power is available from an electric power distribution grid G. For example the reversible electric machine 119 can operate in the motor mode at night time so that electric power from the grid G is converted into thermal power accumulated in the thermal energy storage unit and in pressure energy stored in the form of compressed air in the compressed air storage device 117. When no power is available from the electric power distribution grid G, the first clutch 121 can be disengaged and the reversible electric machine 119 can remain at still stand.
If additional power is required from the electric power distribution grid G, the reversible electric machine 119 can be switched in the generator mode and connected, through a second clutch 123, to an expander 125. Compressed air from the air storage device 117 can then be delivered through the thermal energy storage unit to the expander 125. The compressed air from the air storage device 117 is heated up to for example 650° C. in the thermal energy storage unit by exchanging heat with the heat storage medium of the thermal energy storage unit 113. Compressed and heated air is expanded in the expander 125, which converts at least part of the power available in the compressed and heated air flow into useful mechanical power, which drives the reversible electric machine 119, thus producing electric power that is finally injected into the electric power distribution grid G.